Thermoplastic elastomer, composition, gasket, molded gasket and structure for sealing between two members

ABSTRACT

According to the present invention, provided is a thermoplastic elastomer composition, comprising an ethylene-propylene-nonconjugated diene ternary copolymer or an ethylene-propylene binary copolymer, a crystalline polyolefin resin, a non-aromatic softening agent, and an organic peroxide, in which the crystalline polyolefin resin has from 0.1 g/10 min. to 100 g/10 min. of melt flow rate which is measured in accordance with JIS K7210 under conditions of 230° C. and 21.18 N and is contained in an amount of from 10 to 150 parts by weight every 100 parts by weight of the copolymer; the non-aromatic softening agent has a kinetic viscosity of 300 mm 2 /s or more at 40° C. and is contained in an amount of from 20 to 150 parts by weight every 100 parts by weight of the copolymer; the organic peroxide is contained in an amount of from 0.1 to 10 parts by weight every 100 parts by weight of the copolymer; and hardness measured by a JIS type A durometer is from 30 to 70 degrees. According to the present invention provided are a thermoplastic elastomer composition, a gasket, a molded gasket, and a sealing structure between two members, in which a quality such as sealability has been improved, and, further, provided are a molded gasket, and a sealing structure between two members, in which a production process has been simplified.

TECHNICAL FIELD

Present invention relates to a thermoplastic elastomer, a gasket, amolded gasket and a sealing structure between two members.

BACKGROUND ART

Conventionally, a gasket has been used as a sealing member for sealing aspace between a main body and a cover of a casing which constitute aframe of each of electronic devices, for example, a hard disk drive of acomputer.

FIG. 4 is a schematic diagram showing a main body and a cover of acasing of a hard disk drive to which this type of gasket is attached ina manner separate from each other.

As shown in FIG. 4, a frame of the hard disk drive 101 comprises acuboid casing main body 110 deprived of a top face, a cover 120constituting the top face of the casing main body 110, and a gasket(sealing member in an endless state) 130 which is interposed between thecasing main body 110 and the cover 120 to seal a space between the twomembers 110 and 120.

In recent years, along with a trend of down-sizing and higherperformance of electronic devices, it is required to allow sizes andthickness of components thereof to be smaller. When the componentsbecome smaller in size, since assembling workability in a productionprocess is deteriorated, it is required to integrate various types ofcomponents or allow them to be a composite. At the same time,enhancements of performances of required characteristics (outgassingproperty, sealability, and quality) are required.

The gasket for an electronic memory device, particularly, hard diskdrive, is attached such that a rubber singular body, or a foamedpolyurethane sheet is inserted in a metallic cover made of, for example,stainless steel or aluminum. Since attaching work is favorably performedby integrating a rubber material (mainly, fluorocarbon rubber) into themetallic cover made of, for example, stainless steel, it has beenproposed to bond rubber to a metal by using an adhesive (Japanese PatentNo. 2517797).

However, in a method described in the above patent, the rubber in agasket shape is previously vulcanize-molded in a separate step and,then, bonded to the metallic cover by using the adhesive. This method islong in production process and complicated. Actually, a vulcanizing steprequired several minutes and, since the gasket after being vulcanizedwas thin and liable to be broken and, also, apt to catch dirt or thelike, it was necessary to perform rinsing several times or conduct aselection work before assembling.

On the other hand, a gasket material comprising a styrene-type elastomerhas been proposed (Japanese Patent No. 2961068), in which it is shownthat a process can be simpler than that of the rubber material becauseit is not necessary to perform a vulcanizing step and a cost reductioncan be realized because the material can be recycled.

In a technique as described in this patent, unless the gasket which isthin, soft, and liable to be adhered is previously fixed by one measureor another, workability becomes extremely inferior at the time ofactually assembling the hard disk drive.

In this technique, as a countermeasure, a gasket comprising astyrene-type elastomer is previously formed on an article called as aframe by injection molding and, then, the thus-formed gasket is attachedto between a casing such as a hard disc drive and a cover to beintegrated thereamong; however, after all, a third article called as theframe comes to be required.

Further, in recent years, the gasket tends to be exposed to heatgenerated along with a higher performance (higher revolutions) of thehard disk drive or in a using environment of higher temperature(particularly, 80° C. or more) by being adopted as an in-vehiclearticle.

In these cases, it can be said that performance of a conventionalstyrene-type elastomer has reached the limit thereof.

Namely, when the gasket is clamped for a long period of time under ahigh temperature, sealability comes to be insufficient due to apermanent deformation thereof.

DISCLOSURE OF THE INVENTION

The present invention has been achieved to solve these problems ofconventional techniques and has an object to provide a thermoplasticelastomer composition, a gasket, a molded gasket, and a sealingstructure between two members in which a quality such as sealability hasbeen improved, and, further, to provide a molded gasket and a sealingstructure between two members in which a production process has beensimplified.

In order to attain the aforementioned object, in the present invention,a thermoplastic elastomer composition, comprising anethylene-propylene-nonconjugated diene ternary copolymer or anethylene-propylene binary copolymer, a crystalline polyolefin resin, anon-aromatic softening agent, and an organic peroxide, is characterizedin that the crystalline polyolefin resin has from 0.1 g/10 min. to 100g/10 min. of melt flow rate which is measured in accordance with JISK7210 under conditions of 230° C. and 21.18 N and is contained in anamount of from 10 to 150 parts by weight every 100 parts by weight ofthe copolymer;

the non-aromatic softening agent has a kinetic viscosity of 300 mm²/s ormore at 40° C. and is contained in an amount of from 20 to 150 parts byweight every 100 parts by weight of the copolymer;

the organic peroxide is contained in an amount of from 0.1 to 10 partsby weight every 100 parts by weight of the copolymer; and

hardness measured by a JIS type A durometer is from 30 to 70 degrees.

Preferably, the crystalline polyolefin resin is contained in an amountof 100 parts by weight every 100 parts by weight of the copolymer.

Preferably, compression set measured in accordance with JIS K6262 after168 hours of standing time at 100° C. is 50% or less.

A gasket is characterized by being formed by using the aforementionedthermoplastic elastomer composition as a material.

A molded gasket constituting a cover member is characterized in that theaforementioned thermoplastic elastomer composition is integrally moldedtogether with a metal sheet as a gasket.

A sealing structure between two members constituted such that a metalsurface of one member and a surface of the other member face to eachother while interposing a gasket therebetween is characterized by beingconstituted such that the gasket which is formed by injection moldingthe aforementioned thermoplastic elastomer composition on a metalsurface of one member coated with an adhesive is pressed against asurface of the other member.

A gasket, which is interposed between a surface of a first member and asurface of a second member that face to each other and is adhered to thesurface of the first member such that it seals a space between the twomembers, is characterized in that, as a cross-sectional shape of thegasket in a width direction, length H0 in a direction in which the twomembers face to each other and width W0 of an adhesion face against thesurface of the first member have a relation of “H0/W0≧0.8” therebetweenand, further, as a cross-sectional shape of the gasket in the widthdirection, a base portion arranged on the side of the surface of thefirst member and a projection portion which is formed in a stateprojected from the base portion and comprises a tip end of a curved facepointing toward the surface of the second member are provided and, stillfurther, a curvature radius R of the tip end of the curved face is 0.1mm or more.

Furthermore, the aforementioned constitutions may be combined thereamongas far as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective diagram of a casing constituting aframe of a hard disk drive in which a gasket is applied according to anembodiment of the present invention;

FIG. 2 is a cross-sectional diagram showing a cross-section of a gasketaccording to each of various embodiments of the present invention and acomparative example;

FIG. 3 is a cross-sectional diagram showing a cross-sectional shape of agasket according to an embodiment of the present invention; and

FIG. 4 is an exploded perspective diagram of a casing constituting aframe of a hard disk drive.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   1, 101 Hard disk drive (casing)-   10, 110 Casing main body-   20, 120 Cover-   30, 130 Gasket-   31 Base end face-   32 Tip end face-   33 Base portion-   34 Projection portion-   R Curvature radius-   H0 Length of gasket in a cross-sectional state-   H1 Length of gasket in a cross-sectional state when being compressed-   W0 Width of adhesion face-   W1 Width of intermediate portion

BEST MODE FOR CARRYING OUT THE INVENTION

The present inventors have studied for obtaining a molded gasket whichdoes not need such a third member as described in Japanese Patent No.2961068 and is capable of attaining improvements of quality such as animprovement of high temperature performance, simplification of aproduction process (facilitation of production), an improvement ofsealability, and an improvement of an outgassing property. As a result,the present inventors have found that a production can be easilyperformed by allowing an adhesive to be contained between a metal sheetand a gasket, allowing a gasket to comprise a specified thermoplasticelastomer composition, and performing integral molding by means ofinjection molding while a metal sheet coated with an adhesive beinginserted. Further, the present inventors have found that compression setis improved by allowing the thermoplastic elastomer composition to bepartially cross-linked and, accordingly, sealability and the like undera high temperature is improved.

Still further, it has been found that, depending on a gasket having aspecific constitution, when the gasket is in a using state, a projectionportion is pressed against a surface of a second member and,accordingly, the gasket is compressed in a highly reproducible andcomparatively constant shape and, therefore, a large reaction forceagainst the surface of the second member is generated; hence, asufficient sealability between a surface of a first member and thesurface of the second member is secured for a long period of time.

Hereinafter, a constitution of an integrated gasket with a cover for ahard disk drive (hereinafter also referred to simply as “cover”) as amolded gasket according to an embodiment of the present invention isdescribed.

FIG. 1 is a schematic diagram showing a state in which a casing mainbody and a cover of a hard disk drive to which a gasket according to thepresent embodiment is attached are separated from each other.

In a same manner as in a previous conventional technique, a frame of thehard disk drive 1 according to the present embodiment comprises a cuboidcasing main body 10 deprived of a top face, a cover 20 constituting thetop face of the casing main body 10. As for the cover 20, a metal sheet,such as an aluminum sheet, a plated aluminum sheet, a stainless steelsheet or a laminated damping steel sheet made of stainless steel isused. On a rear face (face on the side facing to casing main body 10) ofthe cover 20, a gasket (endless sealing member) 30 which is interposedbetween the casing main body 10 and the cover 20 and seals a spacebetween the two members 10 and 20 is adhered in assembling the case.

The gasket 30 is composed of a thermoplastic elastomer composition,which comprises,

-   a material A: an ethylene-propylene-nonconjugated diene ternary    copolymer or an ethylene-propylene binary copolymer;-   a material B: a crystalline polyolefin resin having a melt flow rate    (MFR) (in accordance with JIS K7210; 230° C., 2.16 kg load (21.18    N)) of from 0.1 g/10 min. to 100 g/10 min;-   a material C: a non-aromatic softening agent having a kinetic    viscosity at 40° C. of 300 mm²/S or more; and-   a material D: an organic peroxide and which contains, based on 100    parts by weight of the material A, from 10 to 150 parts by weight,    preferably 100 parts by weight of the material B,

from 20 to 150 parts by weight of the material C, and

from 0.1 to 10 parts by weight of the material D, and which is partiallycross-linked and is adjusted to have a hardness of from 30 to 70 degrees(in accordance with JIS K6253; type A durometer) .

On the rear face of the cover 20, a liquid adhesive based on a modifiedolefin resin or a styrene-butadiene rubber is applied and, then, acomposition which becomes a material for the gasket 30 is injectionmolded on the face thus-applied with the adhesive and, as a result, thecover 20 and the gasket 30 are instantaneously integrally molded.

Next, each of materials constituting the thermoplastic elastomercomposition according to the present embodiment is described. (Copolymer(hereinafter referred to also as “copolymer rubber”))

As for copolymers, mentioned is an ethylene-propylene-nonconjugateddiene ternary copolymer or an ethylene-propylene binary copolymer.

The ethylene-propylene-nonconjugated diene ternary copolymer refers to acopolymer obtained by copolymerizing monomers of ethylene, propylene,and a nonconjugated diene thereamong, or a copolymer obtained bycopolymerizing an ethylene-propylene binary copolymer and anonconjugated diene therebetween. As for theethylene-propylene-nonconjugated diene ternary copolymer rubber, it ispreferable that an ethylene content thereof is in the range of from 50to 80% by weight and an iodine number thereof is in the range of from 10to 25.

As for the ethylene-propylene binary copolymer rubber, it is preferablethat an ethylene content thereof is in the range of from 10 to 25% byweight and MFR thereof is in the range of from 3 g/10 min. to 30 g/10min.

In this case, as for the nonconjugated diene, dicyclopentadiene,1,4-hexadiene, dicyclooctadiene, methylene norbornene, ethylidenenorbornene or the like is used.

As for the ethylene-propylene-nonconjugated dience ternary copolymerrubber, practically, various types of EPDM which are available in themarket can be used.

(Crystalline Polyolefin Resin)

As for the crystalline polyolefin resin, a polypropylene-type resin ispreferable. The polypropylene-type resin, which is a thermoplastic resinobtained by polymerizing propylene in the presence of a catalyst, is acrystalline polymer having, for example, an isotactic or a syndiotacticstructure or a copolymer of the crystalline polymer and a small amountof an α-olefin (for example, ethylene, 1-butene, 1-hexene, or4-methyl-1-pentene).

Among such copolymers, a copolymer in which the MFR (in accordance withJIS K7210; 230° C.; and 2.16 kg load (21.18 N)) is from 0.1 g/10 min. to100 g/10 min. and the crystallinity is from 20 to 70% is preferable.

On this occasion, when the MFR is unduly smaller than 0.1, flowabilityis deteriorated and, accordingly, a targeted moldability can not beobtained. Whereas, when the MFR is unduly larger than 100, a sufficientphysical property cannot be obtained.

Further, in order to obtain a desired moldability or hardness, apolypropylene-type resin is indispensable. However, when thepolypropylene-type resin is unduly large in amount, the hardness becomeshigh while, when the polypropylene-type resin is unduly small in amount,the flowability becomes deteriorated and, accordingly, it becomesdifficult to perform injection molding.

Therefore, it is preferable that the crystalline polyolefin resin iscontained in an amount of from 10 to 150 parts by weight every 100 partsby weight of the copolymer.

(Softening Agent)

As for the softening agent, a softening agent which is used in anordinary rubber or thermoplastic elastomer is permissible so long as itis a non-aromatic softening agent having a kinetic viscosity at 40° C.of 300 m²/s or more. Examples of such softening agents includepetroleumoil-type softening agents such as process oil, a lubricant andparaffin-type oil; and fatty oil-type softening agents such as castoroil, linseed oil, rapeseed oil, and cocoanut oil.

Further, when the kinetic viscosity of the softening agent is unduly low(100 mm²/s or less) , a material having a low molecular weight becomeslarge in amount which comes to be detected as an outgas.

Still further, when the softening agent is unduly large in amount,outgassing becomes large while, when it is unduly small in amount, itcan not be adjusted to have a desired hardness; these cases are notfavorable. A preferable amount of the softening agent is from 10 to 200parts by weight.

(Organic Peroxide)

As for cross-linking agents, organic peroxides are primarily favorable.Examples of the organic peroxides include dicumyl peroxide,di-tert-butyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexanebenzoyl peroxide, cumyl peroxide,1,3-di(tert-butylperoxy)isopropylbenzene, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, m-toleyl peroxide, dipropyonyl peroxide. Theorganic peroxides are each used in the range of approximately from 0.1to 10 parts by weight and, preferably, approximately from 0.5 to 8 partsby weight every 100 parts by weight of the copolymer rubber.

Further, when it becomes unduly larger than 10 parts by weight inamount, cross-linking becomes tight and, accordingly, elasticity(mainly, elongation) as the elastomer becomes small and, as a result, afunction as a gasket is impaired. Still further, when it is undulysmaller than 0.1 part by weight in amount, the cross-linking is scarcelyperformed and, accordingly, compression set becomes large, namely,sealability becomes deteriorated.

And, in the thermoplastic elastomer composition containing theaforementioned constituents according to the present embodiment,hardness measured by a type A durometer in accordance with JIS K6253 isadjusted to be from 30 to 70 degrees.

When the hardness is unduly larger than 70 degrees, a reaction force atthe time of attaching the cover-integrated gasket into the main bodybecomes large and, accordingly, a cover deformation or the like isgenerated and, as a result, sealing can not be fully performed; hence,sealability as a gasket is inferior.

On the other hand, when the hardness becomes unduly smaller than 30degrees, the outgassing becomes large. Further, it becomes necessary topay attention to handling because it is likely to be broken or easy tobe adhered. The most preferable hardness is from 40 to 60 degrees.

Still further, the hardness can be adjusted by changing an amount of thecrystalline polyolefin resin to be compounded, or an amount of thenon-aromatic softening agent to be compounded. Basically, the gasket canbe harder by increasing the amount of the crystalline polyolefin resinto be compounded, while it can be softer by increasing the non-aromaticsoftening agent to be compounded and, therefore, by taking awell-balance between the aforementioned two incidents, a desiredhardness can appropriately be adjusted.

In the composition according to the present embodiment, a scalyinorganic filler which is compounded in an ordinary rubber orthermoplastic elastomer, specifically, clay, diatomaceous earth, talc,barium sulfate, calcium carbonate, magnesium carbonate, a metal oxide,mica, graphite, or aluminum hydroxide can be used.

Further, a solid filler in powder form, for example, any of varioustypes of metal powder, glass powder, ceramic powder, granulated orpulverized polymer; an antiaging agent such as an amine or a derivativethereof, any of imidazoles, any of phenols or derivatives thereof; andany of waxes can be used in a range which does not impair theperformance.

Further, various types of additives, such as a stabilizer, an adhesionenhancer, a parting agent, a pigment, a flame-retardant, or a lubricantcan be added. Still further, in order to improve abrasion resistance,moldability or the like, a small amount of thermoplastic resin or rubbercan be added. Furthermore, in order to improve strength or rigidity, ashort fiber or the like can be added in a range which does not impairthe performance.

The cross-linking method of the thermoplastic elastomer compositionaccording to the present embodiment is not particularly limited andknown methods can be used. Among these methods, a kinetic cross-linkingmethod is preferably used in view of efficiency.

The production method of the thermoplastic elastomer compositionaccording to the present embodiment is not particularly limited and thethermoplastic elastomer composition can be produced by a known method.

The thermoplastic elastomer composition can easily be produced byperforming melt-kneading by means of, for example, a heat-kneader, suchas a single-screw extruder, a twin-screw extruder, a roll, a Bumbury'smixer, a brabender, a kneader, or a high shearing-type mixer and, then,by adding a cross-linking agent such as an organic peroxide, across-linking auxiliary, or the like while simultaneously mixing thesenecessary components and, thereafter, by heat-melt-kneading.

Further, a thermoplastic material in which a polymeric organic materialand a softening agent are kneaded is previously prepared and, then, thethus-prepared material is added to at least one polymeric organicmaterial which is same with or different from that used in the above tothereby produce the thermoplastic elastomer composition.

The thus-produced thermoplastic elastomer composition is molded into adesired shape by a known method, such as injection molding or extrusionmolding and, then, the resultant mold can be used as a gasket.

Next, the metal sheet is explained.

In the present embodiment, it is favorable that, as the metal sheet, analuminum sheet or a plated aluminum sheet, a stainless steel sheet or alaminated damping steel sheet made of stainless steel is used. Any onethese sheet is used such as a cover for hard disk drive. Further, themetal sheet is not limited to these metal sheets and may appropriatelybe determined depending on specifications of products.

Still further, the molded gasket according to the present embodiment isan integrally molded product of the gasket produced from theaforementioned thermoplastic elastomer composition, and the metal sheet.

Particularly, the metal sheet is coated with an adhesive which is basedon a modified olefin-type resin and made into a liquid state or based onstyrene-butadiene rubber and made into a liquid state and, then,inserted into a mold and, thereafter, injection molded to thereby allowthe gasket and the metal sheet to be surely and instantaneouslyintegrated. In a case in which the adhesive is not used, since there isa risk of generating peeling-off at the time of molding, it ispreferable that the metal sheet is coated with the adhesive and, then,integrally molded.

Next, the adhesive is explained.

As for such adhesives, an adhesive in which a polar group (such asmaleic acid anhydride, acrylic acid, an epoxy group, or a hydroxylgroup) is graft-polymerized to a side chain of a polyolefin-type resinto allow the polyolefin-type resin to be modified and, then, thethus-modified polyolefin-type resin is dissolved in an aromatic oraliphatic organic solvent to be in a liquid state or in a dispersionstate, another adhesive in which styrene-butadiene rubber is dissolvedin an aromatic or aliphatic solvent to be in a liquid state, or amixture thereof is favorable. As for application methods of theadhesive, an optimal method may be selected from a dipping coating, aspray coating, a screen printing, a brushcoating, a stamping method andthe like, as needed.

When an epoxy type or a cyanoacrylate type is used as the adhesive,integral molding can be performed but, after the integral molding, asufficient adhesive strength can not be obtained such that the gasket iseasily peeled off over the cover and, therefore, other adhesives thanthese adhesives may be used.

By favorably using the aforementioned thermoplastic elastomercomposition, a specified gasket according to the embodiment of thepresent invention can be produced.

A cross-sectional shape of the gasket takes at least one of shapes A,Band C shown in FIG. 2. These gaskets have meaningfully excellentperformance from the standpoint of sealability, water permeability, anadhesive property and the like. The term “cross-sectional shape” as usedherein means a shape of a cross-section cut in a width direction of thegasket formed in rope form.

FIG. 3(a) shows a state in which the gasket having the shape C isintegrally formed with a rear face of the cover 20. Further, FIG. 3(b)shows a state (state in use) in which a gasket sample having the shape Cin a same manner as in the above is interposed between the cover 20 andthe casing main body 10 to thereby seal a space between the two members20 and 10.

As is shown in FIG. 3(a), in the gasket having the shape C, width W1 ofan intermediate portion between a base portion 33 and a projectionportion 34 is formed so as to be shorter than width W0 of an adhesionface against the rear face of the cover 20 and a tip end face 32 forms acurve having a specified curvature radius R. In a state in use (see FIG.3(b)), the projection portion 34 is pressed against the casing main body10 and compressed in a highly reproducible and comparatively constantshape and, accordingly, a large reaction force F against the casing mainbody 10 is generated. Therefore, a high sealability is secured betweenthe two members 10 and 20 for a long period of time.

Further, inventors have confirmed that the gasket adopted the shape A orB performs a function in accordance with that in a case in which theshape C is adopted.

On this point, the inventors have exerted an intensive study and, as aresult, found that, as a cross-sectional shape of the gasket in a widthdirection, length H0 in a direction in which the two members 10,20 faceto each other and width W0 of an adhesion face against the cover 20 havea relation of “H0/W0≧0.8” therebetween and, further, as across-sectional shape of the gasket in the width direction, a baseportion 33 arranged on the side of a rear face of the cover 20 and aprojection portion 34 which is formed in a state projected from the baseportion 33 and comprises a curved face (tip end face) 32 pointing towarda face of a periphery of an opening portion of the casing main body 10are constituted and, still further, conditions in which a curvatureradius R of the tip end face 32 is approximately 0.1 mm or more(preferably 0.2 mm or more), and a compression rate [100×(H0−H1)/H0] (%)of the gasket in use is approximately 20% or more are adopted along withthe thermoplastic elastomer composition and, then, under thesecircumstances, meaningfully excellent performance in sealability, waterpermeability, moldability and the like can be obtained.

Further, in the aforementioned embodiment, a constitution in which theadhesion face of the gasket is arranged on the side of the cover isadopted but a constitution in which the adhesion face of the gasket isarranged on a face of a periphery of an opening portion of the casingmain body may be adopted.

The gasket according to the present embodiment as described aboveprevents, particularly, vapor or dust from entering into electronicdevices and, further, can favorably be used as a gasket for the harddisk drive in the field of a precision devices which require a lowoutgassing property and a high dust prevention property.

Particularly, it is favorably used for the hard disk drive having a highperformance (high revolution), or the hard disk drive which is used in ahigh temperature environment as an in-vehicle article and the like.

Further, other than the aforementioned applications, the gasket canfavorably be used in any of members which require air-tightness invarious types of products as an ordinary gasket or packing.

EXAMPLE

An embodiment according to the present invention will be describedbelow.

Various types of samples were prepared by compositions shown in Table 1and various types of evaluations were performed on the items describedbelow.

Ethylene-propylene-nonconjugated Diene Ternary Copolymer Rubber

-   Sample A: EPDM (manufactured by Mitsui Chemical Co., Ltd.; trade    name: EPT3045)

Crystalline Polyolefin Resin

-   Sample B: polypropylene-type resin (manufactured by Idemitsu Kosan    Co., Ltd.; trade name: J700GP)

Softening Agent

-   Sample C: paraffin-type oil (manufactured by Idemitsu Kosan Co.,    Ltd.; trade name: Diana process oil PW380)

Organic Peroxide

-   Sample D: dicumyl peroxide (Nippon Oils & Fats Co., Ltd.; trade    name: Percumyl D)

Adhesive

-   Modified olefin-type resin adhesive (manufactured by Mitsui Chemical    Co., Ltd.; trade name: Unistol R120K)    (Preparation of Sample)

A thermoplastic elastomer composition was obtained such that apredetermined amount of a compound as shown in Table 1 was measured and,then, subjected to mix-extrusion by a twin-screw extruder (manufacturedby Kabushiki Kaisha Kobe Seikosho; Hiper KTX46) under conditions of aset temperature of from 180 to 210° C. and a revolving speed of 150 rpm.

The resultant material was treated by an injection molding machine(Kawaguchi Tekko K.K.; KM-80) under conditions of a set temperature offrom 180 to 210° C., an injection speed of 0.5 second, an injectionpressure of 100 MPa, and a cycle time of 30 seconds to thereby mold atest sheet (150×150×2 mm) which is, then, subjected to tests forhardness, compression set, an outgassing property, and waterpermeability.

Further, a member in which an adhesive was applied on an aluminum sheet(provided from 2 to 5 μm of electroless nickel plating: hereinafterreferred to also as “cover”) which has previously been imparted with acover shape (for example, a shape of the cover 20) was inserted into ametal mold and treated at an injection speed of 0.5 second, an injectionoutput of 100 MPa, and a cycle time of 30 seconds to thereby form agasket on a surface of the cover. The resultant cover-integrated gasketwas put on a sealability test, an adhesive property test, and amoldability test.

(Cross-sectional Shape of Gasket Sample)

In FIG. 2, a cross-sectional shape of each of gasket samples applied toExamples 1 to 7, Comparative Examples 2, 3, and 5 to 7 described inTable 1 is shown. The term “cross-sectional shape” as used herein meansa shape of a cross-section cut in a width direction of the gasket whichis formed in rope form.

Being common to shapes A to E, a gasket (30) comprises a base end face31 which is adhered to a rear face of a cover 20 and a tip end face 32which is pressed against a face of a periphery of an opening portion ofa casing main body 10. The gasket (30) is interposed between a rear face(surface of a first member) of a cover 20 and a face (surface of asecond member) of the periphery of the opening portion of the casingmain body 10 which face to each other and keeps an inside of a hard disk(casing) 1 in a hermetical state by sealing a space between the twomembers 10 and 20.

On this occasion, H0 means length of the gasket in the direction inwhich two members face to each other. Further, W0 means width W0 of anadhesion face against a surface of the first member. Still further,particularly, an entire shape of each of the shapes A to C is dividedinto a base portion 33 which is arranged on the side of a surface of thefirst member and a projection portion 34 which is arranged on the sideof a surface of the second member. The projection portion 34 comprises acurved face (tip end face 32) pointing toward the surface of the secondmember and has a shape which is formed in such a state as projected fromthe base portion 33.

(Evaluation Method)

(1) Hardness

Three test sheets each having a thickness of 2 mm were put one on top ofanother and, then, measurements were conducted in accordance with JISK6253.

(2) Compression Set Test

Compression set test was conducted in accordance with JIS K6262.Evaluation of the compression set after 168 hours at 100° C. wasperformed in accordance with the following evaluation criterion:

less than 50%: O

50% or more: ×

(3) Outgassing Property

A test piece of 50×3×2 mm in rectangular form was subjected to thermalextraction at 120° C. for one hour and a volume (μg/g) of the outgas wasmeasured.

In this test, evaluation was performed in accordance with the followingevaluation criterion:

Outgas volume: less than 50 (μg/g): O

Outgas volume: 50 (μg/g) or more: ×

In this occasion, the gasket in which the outgas volume shows 50 (μg/g)or more is not appropriate as a gasket for hard disk.

(4) Water Permeability Test

10 cc of distilled water was poured in an SUS container in cylindricalform (inner diameter: 27 mm; depth: 50 mm) and, then, a test piece whichwas adjusted to have a diameter of 30 mm and a thickness of 1 mm wasinterposed and, thereafter, fixed by a hollow cover (inner diameter ofopening portion: 27 mm) made of SUS. Water permeability coefficient(g·mm/cm²·24 h) was determined from data after 100 hours at 70° C.

In this test, evaluation was performed in accordance with the followingevaluation criterion:

Water permeability coefficient: less than 5×10⁻³ (g·mm/cm²·24 h): O

Water permeability coefficient: 5×10⁻³ or more (g·mm/cm²·24 h): ×

In this occasion, the sample in which the water permeability coefficient(g·mm/cm²·24 h) is 5×10⁻³ or more is not appropriate as a gasket forhard disk.

(5) Moldability Evaluation

In injection molding of the product, evaluation was performed inaccordance with the following evaluation criterion:

Absence of defect: O

Presence of defect: ×

The term “defect” as used in this occasion means incapability of moldinginto a designated product shape, in which deformation, surface sink,surface defect, weld mark, shortshot, burr or the like is generated or aphenomenon in which a cover can not be integrally molded occurs.

(6) Evaluation of Sealability

A gasket which is integrally formed together with a cover is mounted onan actual machine-type leakage test device and subjected to a heatingtreatment at 80° C. for 168 hours and, then, cooled down to roomtemperature and, thereafter, exposed to a positive pressure of 5 kPa for30 seconds from inside the test device and, then, after 15 seconds, wastested as to whether or not leakage is generated. In this test,evaluation was performed in accordance with the following evaluationcriterion:

Absence of leakage: O

Presence of leakage: ×

In a case in which compression set of a gasket material is inferior orin another case in which there is a defect in a gasket shape, leakage isgenerated.

(7) Evaluation of Adhesive Property

A through-peeling of about 1 mm was formed on an adhesion face of agasket which was integrally formed together with a cover and, then, awire made of SUS was allowed to go through such peeled portion and,thereafter, a vertical tensile load was applied to the wire. When lengthof the peeled portion became 10 mm by expansion, the load (peeling load)was measured. In this test, evaluation was performed in accordance withthe following evaluation criterion:

Peeling load: 100 (kPa) or more: O

Peeling load: less than 100 (kPa): ×

The gasket in which the peeling load shows 100 (kPa) or more assures asufficiently high adhesive strength even in an actual applicationenvironment.

The test results are shown in Table 1. TABLE 1 Component ofthermoplastic elastomer compound Composition (content: part by ExampleComparative Example weight) 1 2 3 4 5 6 7 1 2 3 4 5 6 7 (1) Sample A(EPDM) 100 100 100 100 100 100 100 100 100 100 100 100 100 — (2) SampleB (PP) 25 25 25 40 15 25 25 5 160 25 25 25 25 — (3) Sample C 100 80 60100 20 100 100 100 100 10 10 100 100 — (softening agent) (4) Sample D 22 2 2 2 2 2 2 2 2 2 2 2 — (cross-linking agent) Cross-sectional A A A AA B C A A D E A shape of gasket R of projection 0.2 0.2 0.2 0.2 0.2 0.250.17 0.2 0.2 — 1.0 0.2 portion H0/W0 1.12 1.12 1.12 1.12 1.12 1.12 1.121.12 1.12 0.8 1.0 1.12 W1/W0 0.5 0.5 0.5 0.5 0.5 0.74 0.5 0.5 0.5 1.01.0 0.5 (1) Hardness 45 52 60 69 50 45 45 93 85 45 45 47 (durometer typeA) (2) Compression set ∘ ∘ ∘ ∘ ∘ — — x x — — x (3) Outgassing ∘ ∘ ∘ ∘ ∘∘ ∘ ∘ ∘ ∘ ∘ ∘ property (4) Water ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ permeability(5) Moldability ∘ ∘ ∘ ∘ ∘ — — ∘ ∘ — — ∘ (6) Sealability ∘ ∘ ∘ ∘ ∘ ∘ ∘ xx x x x (7) Adhesive ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ 47 property* As Comparative Example 7, a styrene-type thermoplastic elastomer[ELASTOMER-AR-540; manufactured by ARONKASEI CO., LTD.] was used.

As described above, the thermoplastic elastomer composition having aconstitution as shown in the present embodiment has excellentcharacteristics in sealability, an outgassing property, waterpermeability, an adhesive property, moldability for a long period oftime under a high temperature and is capable of being constructed as acover-integrally-molded gasket having an extremely admirableperformance. On the other hand, gaskets produced under such conditionsas shown in Comparative Examples have a drawback in at least one of theabove-described performances required for a gasket for hard disk drive.

INDUSTRIAL APPLICABILITY

As is described above, by using the gasket according to the presentinvention, it becomes possible to intend to enhance qualities such assealability, an outgassing property, water permeability, an adhesiveproperty, and mold ability for long period of time under a hightemperature. Further, in the molded gasket, it is possible to simplifythe production process.

1-7. (canceled)
 8. A thermoplastic elastomer composition, comprising anethylene-propylene-nonconjugated diene ternary copolymer or anethylene-propylene binary copolymer, a crystalline polyolefin resin, anon-aromatic softening agent, and an organic peroxide, wherein thecrystalline polyolefin resin has from 0.1 g/10 min. to 100 g/10 min. ofmelt flow rate which is measured in accordance with JIS K7210 underconditions of 230° C. and 21.18 N and is contained in an amount of from10 to 150 parts by weight every 100 parts by weight of the copolymer;the non-aromatic softening agent has a kinetic viscosity of 300 mm²/S ormore at 40° C. and is contained in an amount of from 20 to 150 parts byweight every 100 parts by weight of the copolymer; the organic peroxideis contained in an amount of from 0.1 to 10 parts by weight every 100parts by weight of the copolymer; and hardness measured by a JIS type Adurometer is from 30 to 70 degrees.
 9. The thermoplastic elastomercomposition as set forth in claim 8, wherein the crystalline polyolefinresin is contained in an amount of 100 parts by weight every 100 partsby weight of the copolymer.
 10. The thermoplastic elastomer compositionas set forth in claim 8, wherein compression set measured in accordancewith JIS K6262 after 168 hours of standing time at 100° C. is 50% orless.
 11. The thermoplastic elastomer composition as set forth in claim9, wherein compression set measured in accordance with JIS K6262 after168 hours of standing time at 100° C. is 50% or less.
 12. The gasket,which is formed by using the thermoplastic elastomer composition as setforth in claim 8 as a material.
 13. The molded gasket constituting acover member, wherein the thermoplastic elastomer composition as setforth in claim 8 is integrally molded together with a metal sheet as agasket.
 14. The sealing structure between two members constituted suchthat a metal surface of one member and a surface of the other memberface to each other while interposing a gasket therebetween, wherein thesealing structure between two members is constituted such that thegasket which is formed by injection molding the thermoplastic elastomercomposition as set forth in claim 8 on a metal surface of one membercoated with an adhesive is pressed against a surface of the othermember.
 15. A gasket, which is interposed between a surface of a firstmember and a surface of a second member which face to each other and isadhered to the surface of the first member such that it seals a spacebetween the two members, wherein, as a cross-sectional shape of thegasket in a width direction, length H0 in a direction in which the twomembers face to each other and width W0 of an adhesion face against thesurface of the first member have a relation of “H0/W0≧0.8” therebetweenand, further, as a cross-sectional shape of the gasket in the widthdirection, a base portion arranged on the side of the surface of thefirst member and a projection portion which is formed in a stateprojected from the base portion and comprises a tip end of a curved facepointing toward the surface of the second member are provided and, stillfurther, a curvature radius R of the tip end of the curved face is 0.1mm or more.